A research on a magnetic refrigerating technology that provides clean energy and has a high efficiency is proceeded as an environment-friendly refrigerating technology. The magnetic refrigeration material is a magnetic material that produces a magnetocaloric effect under externally applied magnetic field. As shown in patent literature 1, it is known that La(Fe, Si)13 series material produces an improved magnetocaloric effect as a magnetic refrigeration material. In the magnetic refrigeration material disclosed in patent literature 1, it is known that a curie temperature of the magnetic refrigeration material changes by carrying out hydrogen absorption to the magnetic refrigeration material, and the magnetocaloric effect of the magnetic refrigeration material is produced at a room temperature.
As described above, when the hydrogen absorption is carried out to the La(Fe, Si)13 material, a crystal lattice of the La(Fe, Si)13 expands in volume and a dimension of the crystal lattice of the La(Fe, Si)13 increases since hydrogen atoms are absorbed by the crystal lattice of the La(Fe, Si)13. As a result, stress may be easily generated at a grain boundary and a boundary between different compositions. Accordingly, a crack may be easily generated in the material caused by the stress, and it may be difficult to restrict a generation of the crack.
The following will describe an example of a stress generation that causes the crack. La(Fe, Si)13 material includes small amount of alpha iron (α-Fe) that is generated during a sintering process. The sintering process is carried out in order to generate the crystal lattice in the La(Fe, Si)13 material. FIG. 7(a) and FIG. 7(b) are schematic diagrams showing an enlarged cross-sectional view of a part of the magnetic refrigeration material. FIG. 7(a) shows the schematic diagram before a hydrogen absorption is carried out, and FIG. 7(b) shows the schematic diagram after the hydrogen absorption is carried out.
As shown in FIG. 7(a), before the hydrogen absorption is carried out, an α-Fe portion 101 is contacted with a La(Fe, Si)13 alloy portion 103. As shown in FIG. 7(b), after a hydrogen absorption for absorbing hydrogen 107 is carried out to the magnetic refrigeration material, the La(Fe, Si)13 alloy portion 103 absorbs hydrogen and expands as a La(Fe, Si)13H alloy portion 105. On the other hand, the α-Fe portion 101 does not absorb hydrogen 107 and does not expand. As a result, a gap 109 is generated between the α-Fe portion 101 and the La(Fe, Si)13H alloy portion 105, and the gap causes the crack of the magnetic refrigeration material.